In the field of ultra high frequency (UHF) electronics, particularly for systems operating at frequencies above 1 GHz or higher, significant signal attenuation and power loss has been attributable to mechanical connectors between system components. A particularly troublesome connection in the field of UHF electronics is that between transmission lines of different types, for example the connection between microstrip transmission lines and coaxial cable.
Referring to FIGS. 1a and 1b, a conventional connector 2 for connecting coaxial cable to a trace on a substrate or circuit board, where the trace is a conductor in a microstrip transmission line, is illustrated in front and side elevation views. Mounting plate 4 of connector 2 has non-tapped holes 5 through which machine screws may pass to mount connector 2, as will be described hereinbelow. Connector 2 also has threaded end 3 for threadably connecting to conventional coaxial cable, for example of the 50.OMEGA. type.
Contained within connector 2 is an interior wire 7 (see FIG. 1a) which connects on one side to the interior conductor of the coaxial cable, and which terminates, on the other side, as lead tab 9. Surrounding wire 7 within connector 2 are inner insulator portion 6 and extending insulator portion 8. The material of insulator portions 6, 8 is TFE fluorocarbon or other conventional material. Inner insulator portion 6 has a circular cross-section, and terminates at the face of plate 4. Extending insulator portion 8 extends away from plate 4, and has a smaller cross-section than that of insulator portion 6 within plate 4. The length of extending insulator portion 8 corresponds approximately to the thickness of the chassis or wall to which connector 2 is mounted. Lead tab 9 extends from extending insulator portion 8 and is suitable to be soldered to a conventional microstrip transmission line on a patterned substrate. Lead tab 9 either may have a circular cross-section, or alternatively may be a flat tab.
It has been observed that conventional connectors, such as connector 2 of FIGS. 1a and 1b, significantly attenuate UHF electrical signals, particularly as the signal frequency increases above 1 GHz. This attenuation is believed to be due to physical impedance discontinuities resulting from the construction of the connector and the quality of the connection. One such discontinuity is that presented by the reduction in diameter from insulator portion 6 within plate 4 to the smaller diameter of extending portion 8. This step-down in the diameter of the insulating material can attenuate UHF signals, as it presents a discontinuity of the 50.OMEGA. impedance in the system between the microstrip transmission line to which lead 9 connects and the coaxial cable to which threaded end 3 connects. Furthermore, the diameter of the center pin (e.g., wire 7) in conventional connectors steps down in diameter at the same location at which the diameter of the insulating material steps down in diameter, exacerbating the impedance discontinuity. Other discontinuities presented by connector 2 will now be described relative to FIG. 2, which illustrates, in cross-section, the mounting of conventional connector 2 to a conventional chassis.
In FIG. 2, connector 2 is illustrated as mounted to chassis 11 by way of machine screws (not shown) that pass through mounting holes 5 in plate 4 and that thread into tapped holes (not shown) in chassis 11. As a result, plate 4 of connector 2 is located outside of the wall of chassis 11. Extending portion 8 extends through a matching hole in the wall of chassis 11, so that lead tab 9 extends therefrom as shown. Chassis 11 includes a floor portion upon which substrate 13 is placed in the assembly process. Substrate 13 includes microstrip transmission lines thereon, including trace 15 to which connection is to be made by connector 2 in this example. In the best case, lead tab 9 extends from chassis 11 at a height that is slightly above (e.g., 0.002 inches) trace 15 on substrate 13, to allow for subsequent soldering to make a good connection therebetween.
The installation of connector 2 is conventionally performed after substrate 13 is in place within chassis 11. Lead tab 9 and extending insulator portion 8 of connector 2 are inserted into the hole in the wall of chassis 11, and screwed into place by machine screws passing through holes 5 of connector 2 and threading into threaded holes in the wall of chassis 11. After connector 2 is tightened into place, soldering of lead tab 9 to its mating trace 15 is performed in the conventional manner.
Significant problems have been observed that adversely affect the quality and reliability of the connection between lead tab 9 and trace 15, and that contribute to the attenuation of UHF electrical signals communicated through connector 2, according to this conventional construction and method of installation, however. These mechanical problems primarily arise from the tolerances to which the substrate 13 and chassis 11 can be constructed, as will be apparent from the following example.
The relationship between substrate 13 thickness and the position of connector 2 can be considered as follows: EQU t.sub.13 +GAP=h.sub.9 -0.5(t.sub.9)
where t.sub.13 is the thickness of substrate 13, h.sub.9 is the height of the center of lead tab 9 above the floor of chassis 11, and t.sub.9 is the thickness of lead tab 9, with GAP being the gap between the bottom of lead tab 9 and the top of trace 15. By way of example, a typical substrate 13 is manufactured to a thickness specification (including the thickness of trace 15) of 0.202.+-.0.011 inches. In order to make connection to such a substrate, in this example the specification of the position of the center of the hole through the wall of chassis 11 is 0.209.+-.0.003 inches above the floor of chassis 11. Considering lead tab 9 with a thickness of 0.010 inches, the nominal value of GAP will be 0.002 inches in this example, suitable for a high reliability solder connection.
However, the tolerances specified above for substrate 13 thickness and connector 2 position can result in undesirable GAP values, and significant mechanical problems. For example, where substrate 13 is manufactured to its minimum thickness within the specification range, and where the hole in the wall of chassis 11 is at its highest position, the value of GAP in this example will be +0.016 inches. This large gap between lead tab 9 and trace 15 may result in an imperfect solder connection therebetween, or in electrical transmission discontinuity. Especially at UHF frequencies, such a poor connection will significantly attenuate the power of the signals transmitted between the microstrip transmission line and the coaxial cable. In addition, thermal cycling of such a poor solder connection can produce a later life open connection, resulting in system failure after installation and costly corrective action.
The other extreme condition in this example is for substrate 13 manufactured to its maximum specification thickness in combination with the position of connector 2 at its lowest position. In this condition, the value of GAP will be -0.012 inches (i.e., the lower edge of lead tab 9 is below the top of trace 15 by this amount), resulting in a "crash" fit connection as connector 2 is inserted through the wall of chassis 11 with substrate 13 in place. This crash fit can result in lifting of trace 15 by lead tab 9 when it is installed, if lead tab 9 is inserted in such a manner as to peel trace 15 from substrate 13. Lead tab 9 may also bend or break when inserted in such a crash fit connection, resulting in poor solder connection, UHF signal attenuation and, in the worst case, an open connection.
Another consideration in this conventional connection scheme is the thickness tolerance to which the wall of chassis 11 can be machined, relative to the length of extending insulator portion 8. If the wall of chassis 11 is too thin, insulator extending portion 8 will protrude into the interior of chassis 11, pushing against substrate 13 when inserted and possibly preventing connection therebetween. If the wall of chassis 11 is too thick, air will surround lead tab 9 within chassis 11, presenting an impedance discontinuity that will tend to attenuate UHF signals communicated through connector 2. Furthermore, the tolerance of the angle at which the floor of chassis 11 is machined relative to the wall (nominally perpendicular) can also affect the fit of lead tab 9 to trace 15, and present problems similar to those noted hereinabove.
Still another problematic dimensional tolerance is that of the tolerance of the diameter of the hole in the wall of chassis 11 into which extending insulator portion is placed. If the hole is too small, connector 2 cannot be inserted thereinto. If the hole is too large, wire 7 and lead tab 9 will not be symmetrically located within the cross-section of the hole, which can also affect the impedance of the connection, and attenuate UHF signals communicated through connector 2.
The problems caused by the relative tolerances of the substrate and chassis have been previously addressed by tightening the manufacturing specifications of the various components, which of course greatly increases system manufacturing cost of the system due to lower manufacturing yield to the more stringent specifications. Even with tighter dimensional tolerances, however, the problems of excessive gaps, crash fits and impedance discontinuities discussed above have still been observed to a significant extent.
While the above-described problems are present for a single connector into a chassis, virtually every system requires multiple connectors for each chassis, located on more than one wall, each connecting to the same substrate. The provision of multiple connectors on multiple sides of the chassis not only increases the likelihood of a bad connection, but also brings into play other dimensional tolerances such as the angles of the chassis wall to one another, the flatness of the substrate to which connection is made, and the like.
These problems arising from the manufacturing tolerances of the chassis and substrates occur even for chassis formed by precision machining. As a result, the use of less precise, and less costly, manufacturing processes for the chassis (e.g., the use of sheet metal chassis) has been precluded for UHF systems.
It is therefore an object of the present invention to provide a connector which can be connected to a transmission line trace in an adjustable manner.
It is a further object of the present invention to provide such a connector which allows the chassis and substrate to be manufactured to relatively loose tolerances, and thus with low manufacturing costs.
It is a further object of the present invention to provide such a connector which enables the use of low cost chassis materials, such as sheet metal, while still maintaining high quality and high reliability connections.
It is a further object of the present invention to provide such a connector which reduces impedance discontinuities in making a coaxial-to-microstrip connection through a chassis wall.
It is a further object of the present invention to provide such a connector which provides flexibility of mounting both vertically and horizontally, thus accounting for variations in substrate thickness and chassis hole placement, as well as for variations in the location of traces on the substrate.
It is a further object of the present invention to provide an ultra high frequency chassis incorporating such a connector system.
Other objects and advantages will be apparent to those of ordinary skill in the art having reference to the following specification together with the drawings.